1. Field of the Invention
The present invention relates to an optical information recording medium adapted to record information via holography and electronic equipment using that optical information recording medium.
2. Description of the Related Art
Conventionally, the recording density of optical information recording media, exemplified by compact discs (CDs) and digital video discs (DVDs), has been increased mainly by reducing the wavelength of laser light and increasing the numerical aperture (NA) of an objective lens. However, it is thought that these approaches are nearing limitations from technical reasons. Thus, a demand has increased for new means and systems to increase the recording density.
To meet such a demand, research and development has been conducted actively. As one system to be expected, a holographic memory has been proposed. This holographic memory is disclosed in JP-A 2004-265472 (KOKAI) by way of example. As is described in detail in reference (H. J. Coufal et al., “Holographic Data Storage”, Springer, 2000), the holographic memory, which is one of the so-called volume recording systems, records information on a recording medium in the directions of plane and the directions of height utilizing holography. In short, the holographic memory is adapted to direct an information beam produced by spatially modulating laser light with information and a laser beam (reference beam) of the same wavelength as the information beam onto the same portion of a recording medium and record an interference pattern between the information and reference beams in the recording medium. Upon reproduction, only a reference beam is directed onto the recording medium, whereby the information beam at the record time is reproduced and the information is recovered.
The DVD is a so-called plane recording system in which recording marks representing information are formed on the recording surface, while the holographic memory is a volume recording system. For this reason, the holographic memory is expected to achieve higher recording density than the DVD. One way to increase the recording density of the holographic memory is multiplex recording. This involves recording two or more interface patterns in the same location of a recording medium. The multiplex recording techniques include angle multiplexing in which the irradiation angle of laser light is changed and shift multiplexing in which the irradiation position of laser light is changed.
Conventionally, to implement such multiplex recording, movement is needed which involves rotation for angle change. Therefore, a complex mechanism is mounted which is adapted to move and rotate the entire recording medium or the optical section including an objective lens.
In addition, in order to allow users to confirm information recorded on a recording medium, such as a CD or DVD, without reproduction, a system has been proposed which prints or displays characters or patterns of visible size on the label side of the recording medium. For instance, there are a printer adapted to permit direct printing on a side of a recording medium such as a CD or DVD and the Light Scribe system™ proposed by Hewllet-Packerd Development Company which is adapted to make printing using laser light as a function of a CD/DVD recording and playback apparatus.
The use of such apparatus allows the contents of information recorded on a recording medium to be printed on the medium itself and users to recognize a summary of the recorded contents at a glance. Therefore, it is easily expected that such means will also be required of future holographic memories.
On the other hand, there are cases where holograms, such as rainbow holograms or Lippmann holograms, which can be verified with the naked eye are affixed on bills or credit cards in order to increase security.